The proposed interdisciplinary study addresses the debilitating disorder of hereditary sensory and autonomic neuropathy type 1 (HSAN1) and is based on the insight that mutant serine palmitoyltransferase (SPT) activity leads to production of two potentially neurotoxic sphingolipids. We were the first to identify the accumulation of these two atypical desoxysphingoid bases (DSB) in both mutant HSAN1 transgenic mice and HSAN1 patients. The studies bring together expertise in biochemistry (Uniformed Services University, USUHS) and neuroscience and neurogenetics (Massachusetts General Hospital). The project will (1) investigate mutant SPT isozymes in yeast and mammalian cells, (2) perform neurotoxicity studies of the atypical sphingolipids in mice, and (3) assess amino acid supplementation in mutant HSAN1 transgenic mice. The central premise underlying these investigations is that amino acid substrate selectivity alters desoxysphingoid bases and disease severity in HSAN1. Closely related is the premise that one can use behavioral testing and neuropathological examinations to assess the biochemical impact upon the clinicopathological phenotype. We will begin by expressing mutant SPT heterotrimers containing the known HSAN1 mutations in yeast and in mammalian cells (Aim 1). The Km and Vmax of each mutant enzyme for serine, alanine and glycine will be determined using these labeled amino acids. These experiments will allow us to determine whether the mutations differ in their abilities to condense the alternative substrates (alanine and glycine) and inform our studies in rodents. Neurotoxicity studies (Aim 2) will determine the role of DSB in peripheral neurodegeneration of HSAN1 in vivo. Lastly dietary supplementation of serine versus alanine and glycine in mutant HSAN1 transgenic mice will allow us to test behavior and neuropathology in vivo (Aim 3) and prepare for clinical studies in humans. These ex vivo and in vivo studies will give us insight about the synthesis, degradation and neurotoxicity of DSB in HSAN1. The experiments will not only elucidate a new and fundamental insight into neurometabolism but also lead to a potential new treatment for a neurogenetic disorder.